The invention relates to an arrangement for generating an analog-modulated carrier signal having a substantially constant amplitude and a continuous phase .phi.(t) in response to data signals of a given symbol frequency 1/T. The arrangement comprises: a clock circuit synchronized with the symbol frequency for producing a clock signal having a frequency 4q/T, where q is an integer greater than 1; a control circuit comprising an addressing circuit controlled by the clock signal and including an interpolation counter and responsive to a given number of consecutive data symbols for producing addresses having a frequency 4q/T, and a counter controlled by the symbol frequency and responsive to the data symbols for producing phase state numbers which characterize the value modulo-2.pi. of the the phase .phi.(t) at the boundaries of the symbol intervals of the length T. A signal processor is connected to the control circuit and comprises a first read-only memory for storing in addressable locations digital numerical values which are representative of the signals cos .phi.(t) and sin .phi.(t) at the instants determined by the clock signals, the stored values being read from the locations of the first read-only memory under the control of the addressing circuit. The values read are processed to form the analog angle-modulated carrier signal with the aid of digital-to-analog conversion.
Such an arrangement is known from the article by De Jager and Dekker on TFM (Tamed Frequency Modulation) in IEEE Transactions on Communications, Vol. COM-26, No. 5, May 1978, pages 534-542, (see FIG. 15) and from U.S. Pat. No. 4,229,821 (see FIG. 18). In a symbol interval of length T, the phase .phi.(t) of a TFM-signal changes by not more than an amount of .+-..pi./2 rad. and the value modulo-2.pi. of the phase .phi.(t) always remains within this interval in the same phase quadrant [y.pi./2, (y+1).pi./2] with y=0, 1, 2 or 3 at an appropriate choice of .phi.(t) at the reference instant t=0. A possible transition to a different phase quadrant is only effected at the boundaries of the symbol intervals. For a TFM-signal the phase state number is the phase quadrant number y modulo-4. In the prior art arrangement, this phase quadrant number is obtained as a counting position of a modulo-4 up/down counter controlled by the data symbols, and is utilized there as part of the read address for the first read-only memory of the signal processor. The digital numerical values read are converted into two analog signals cos .phi.(t) and sin .phi.(t) by means of two DAC-circuits (Digital-to-Analog Conversion circuits). The two analog signals are applied through two low-pass filters for suppressing undesired signal components at the frequency 4q/T and multiples thereof to an analog quadrature modulation circuit, where they are multiplied by two carriers in phase-quadrature with the aid of two product modulators. The TFM-signal is obtained with the aid of an adder connected to the product modulators.
As the interface between the digital and analog signal-processing section lies immediately after the first read-only memory, these known arrangements have a pronouned hybrid structure and particularly stringent requirements are imposed on the circuit implementation of the analog section, both as regards the equality of the amplitude and the phase characteristics of the two signal paths and the unavoidable d.c. voltage offsets occurring therein. Also, a high regard for the accuracy of the phase quadrature of the two carriers must be maintained to prevent undesired amplitude and phase variations, undesired sidebands and insufficient carrier suppression from occurring in the TFM-signal at the output.
A possible way to avoid the above-mentioned disadvantages is to replace the constitutuent parts of the analog quadrature modulation circuit (product modulators, carrier oscillator and adder) by their digital equivalents which are known per se. These equivalents are arranged for processing signal samples at the rate rq/T of the clock signal and to connect directly the digital quadrature modulation circuit thus obtained to the first readonly memory. The interface between the digital and analog sections is then shifted to the output of the quadrature modulation circuit and consequently only one DAC-circuit is required for obtaining the TFM-signal. For a practical implementation, the predominantly digital structure thus obtained is, however, still not attractive in view of the digital multipliers required in the quadrature modulation circuit which limit the maximum permissible data symbol rate 1/T.
In the article by Chung and Zegers on GTFM (Generalized TFM), published in Philips Journal of Research, Vol. 37, No. 4, 1982, pages 165-177 it is stated (see page 169) that this restriction can be eliminated by choosing an appropriate value for the carrier frequency (for example equal to one quarter of the frequency 4q/T of the clock signal) and by then combining the functions of the first read-only memory and the digital quadrature modulation circuit. The digital numerical values stored in the first read-only memory then represent samples of the analog GTFM-signal and consequently the DAC-circuit can be connected directly to the first read-only memory. The arrangement thus obtained is particularly attractive for monolithic integration and can process a large range of data symbol rates, for example from 2.4 kbit/s to 72 kbit/s. This technique is not limited to the generation of (G)TFM-signals, but alternatively may be used for a wide variety of other modulation methods, such as n-PRCPM (n-ary Partial Response Continuous Phase Modulation) and CORPSK (Correlative Phase Shift Keying) described in the article by Aulin, Rydbeck and Sundberg and the article by Muilwijk in IEEE Transactions on Communications, Vol. COM-29, No. 3, March 1981, pages 210-225 and pages 226-236, respectively, and GMSK (Gaussian Minimum Shift Keying) described in the article by Murota and Hirade in IEEE Transactions on Communications, Vol. COM-29, No. 7, July 1981, pages 1044-1050. For some applications of this technique the required storage capacity may still be objectionable.